Development of X-Ray collimators to identify sources of radiation in devices insulated by large vacuum gaps

The present paper describes the development of a diagnostic system to localize the sources of X-ray in a system insulated by large vacuum gaps. The X-ray emission due to the Bremsstrahlung radiation of electrons, which are unavoidable extracted and accelerated by static electric fields, is one of the main experimental evidences observable during high voltage conditioning of a HVDC system insulated by large vacuum gaps , i.e. with voltages beyond 50-100kV. The technological challenge associated to the achievement of a stable accelerating voltage in MITICA, the prototype of the Neutral Beam Injector for ITER, could be assessed localizing the X-ray sources and identifying the positions of the electron emitters by numerical models. The measurement of the spatial distribution of the X-ray sources could be essential to understand the weak points in term of voltage holding for the MITICA electrostatic accelerator. The adoption of a X- ray collimator embedding a Cerium doped Lutetium-based scintillation crystal has been proposed to cope this target. A prototype of a single pixel collimator with variable geometry shield by lead has been manufactured and tested at the High Voltage Padova Test Facility. High voltage tests with acceleration voltages up to 450kVdc have been carried out to improve the collimator design. The present paper describes the development activity of the single pixel collimator and proposes a conceptual design of a multi pixel system with variable geometry based on the same technology.